1. Field of the Invention
The present invention relates to an exhaust gas recirculation device of an internal combustion engine.
2. Background of the Invention
As a measure to reduce NOx emission from an internal combustion engine, it has been a conventional practice to provide the internal combustion engine with an EGR (Exhaust Gas Recirculation) device.
Thereby, an EGR gas control valve for controlling the flow rate of an EGR gas is conventionally provided in an EGR gas passage which connects an exhaust gas passage to an intake air passage; the opening of the EGR control valve is controlled by a feedback correction device so that the opening of the EGR control valve follows a target opening that is determined in response to the operation condition of the internal combustion engine.
However, in a transient operation condition, it is not always possible to perform the operation under a pertinent target opening condition. In a case where the target opening steeply increases, what they call an overshoot or undershoot of the valve opening occurs for a while after the actual opening level reaches the target opening level; namely, in the overshoot or the undershoot of the valve opening, the actual opening goes or returns to an excessive degree. In this event, when the valve opening overshoots toward an opening excess side, the EGR gas of an excess flow rate over a required flow rate determined on the basis of the target opening is supplied. Thus, there may be a problem that the smoke generated in the gas emitted from the exhaust gas passage increases.
On the contrary, when the valve opening is steeply decreased, the undershoot in which the valve opening decreases toward an opening shortage side so that the EGR gas of an insufficient flow rate below a required flow rate determined on the basis of the target opening. In this event, the flow rate of the EGR gas to be supplied becomes insufficient in comparison with the required flow rate is supplied.
In addition, as shown in FIG. 11, the relationship between the NOx emission amount and the smoke emission amount shows a trade-off relation. Incidentally, the smoke described above includes PM (particular matter).
In this way, it becomes important to provide a technology which shows how the actual valve opening follows the target valve opening without the overshoot or the undershoot after the target opening of the EGR control valve is prescribed.
As for the technology by which the actual valve opening follows the target valve opening without overshoot or undershoot, Patent Reference 1 (JP2832422) and Patent Reference 2 (JP2832423) are known.
Patent Reference 1 (JP2832422) shows a technology by which the overshoot of the EGR control valve opening is prevented so that the integral control action of a PI controller performing a feedback control is not executed (i.e. the integrated value is set as 0) in a situation where the rate of change of the target opening for the EGR control valve is great or in a situation where the deviation between the target opening and the actual opening is increased.
Further, Patent Reference 2 (JP2832423) discloses a control device provided with:
a feedback correction unit by which a feedback correction is performed so that the actual EGR control valve opening follows the target opening; and
a feed-forward correction unit by which a feed-forward correction is performed so that the target opening is achieved,                wherein        
the correction action by the feed-forward correction unit is constrained when the target opening is comparatively smaller in comparison with a case where the target opening is at a greater level; or
the correction action by the feed-forward correction unit is increased when the absolute value of the difference between a target opening and an actual opening of an EGR gas control valve is comparatively greater in comparison with a case where the absolute value of the difference is at a smaller level.